Carbonated steel slag as supplementary cementitious material: Paving the way to full reutilization

Steel slag (SS), one of the most common alkaline solid wastes, faces ongoing challenges in achieving satisfactory reutilization rates. Carbonated steel slag (CSS), produced through pre-carbonation treatment, is gaining attention as an emerging supplementary cementitious material (SCM). Cement incorporating finer CSS particles exhibits satisfied mechanical, environmental, and economic benefits, challenging the stereotype that grinding SS into micron-sized powders negates the carbon sequestration benefits. The hydration of cement incorporating CSS involves several chemical reactions, including cement hydration, pozzolanic reactions, limestone reactions, and weak SS hydration. The interplay between these reactions and the resulting mechanical performance is heavily influenced by the cement replacement ratio and the degree of carbonation of the CSS. Prolonged carbonation of CSS can form an excessively thick calcium carbonate barrier layer, which inhibits the pozzolanic reaction of silica-rich gel, adversely affecting hydration and mechanical performance. In the case of conventional cement-based materials, the single replacement level of cement with CSS can go up to 30 %. With alumina additive or CO 2 -fixed bacteria, or the application in ultra-high performance concrete, the synergistic effect can further enhance secondary hydration reactions of CSS, allowing the cement replacement level to increase up to 60 % with only minor reductions in compressive strength. Comparing CSS with other commercialized SCMs, it is regretful that current findings on long-term performance are still lacking and warrant in-depth investigations.